Autothermal reactors (ATR), partial oxidation reactors and the like are commonly used to produce synthesis gas from light hydrocarbon gases by substoichiometric oxidation with air or oxygen-enriched air. The light hydrocarbon gas and the air or oxygen-enriched air are blended in a proper ratio at a pressure from about 200 to about 500 psig and a temperature from about 700 to 900.degree. F. and substoichiometrically combusted in the ATR to produce synthesis gas having a desired hydrogen to carbon monoxide ratio. Varying amounts of steam can be added to the mixture as desired to vary the hydrogen:carbon monoxide ratio for the production of paraffins, methanol, olefins or the like in a Fischer-Tropsch reactor, for process control or the like. Typically when heavy paraffins are to be produced in the Fischer-Tropsch reaction, the synthesis gas ratio for hydrogen to carbon monoxide is from about 1.5 to about 2.5. Frequently steam is added to the mixture in an amount sufficient to control soot formation when a flame is used. The practice of such technology is considered to be well known to those skilled in the art. The compression and heating of the air or oxygen-enriched air and light hydrocarbon gas is a major expense in the overall process.
Typically the substoichiometric combustion gas product produced at a flame-producing nozzle is passed into a synthesis gas reforming catalyst bed which is typically nickel on alumina or the like to reform the synthesis gas to an equilibrium composition. Alternatively the gaseous mixture may be passed flamelessly into the catalyst bed for reaction on the catalyst to produce an equilibrium mixture of synthesis gas.
The light hydrocarbon gas is typically available from a gas pipeline, subterranean formation or the like at a relatively high pressure typically at least 400 pounds per square inch gauge (psig). In most processes this gas is de-sulfurized and passed directly to the autothermal reactor after heating to a desired temperature which is typically from about 700 to about 900.degree. F. The heating is normally done in a heat exchanger or a fired heater.
The air or oxygen-enriched air (hereafter referred to as air) is typically available at ambient temperature and pressure and is passed to a series of compressors to increase the pressure to about 400 psig. A limiting factor is the increase in air temperature as the air is compressed. Typically the seals in such compressors will withstand temperatures of up to about 400.degree. F. to about 450.degree. F. This equates to single stage of compression ratios of less than about 3.5:1 for each stage. Accordingly in order to achieve the desired pressure up to about 500 psig, it typically takes three stages of compression with intercooling. These compressors may be separate units or they may be formed as a multi-stage axial compressor. In either case it is necessary to use an intercooler, such as an air fan or a water cooler, between stages to reduce the temperature of the compressed air before compression in the next stage. The last stage does not require a cooler since this stream is passed directly to the autothermal reactor, typically via a fired heater or other heat exchanger to raise the temperature of the compressed air to from about 700 to about 900.degree. F. so that the temperature of the compressed air and the light hydrocarbon gas stream are approximately the same.
Since this process typically requires a three-stage compression system and since it requires intercooling which results in the loss of substantial heat which must subsequently be replaced by the use of a fired heater or other heat exchanger, the process contains inefficiencies. These inefficiencies can result in substantial expense. It would be highly desirable if a more economical method was available for the compression and heating of the air supplied to the autothermal reactor.
Accordingly a continuing search has been directed to the development of a more efficient method for compressing and heating the air stream required in the autothermal reactor.